[SPARK-16812] Open up SparkILoop.getAddedJars

## What changes were proposed in this pull request?
This patch makes SparkILoop.getAddedJars a public developer API. It is a useful function to get the list of jars added.

## How was this patch tested?
N/A - this is a simple visibility change.

Author: Reynold Xin <rxin@databricks.com>

Closes #14417 from rxin/SPARK-16812.